The present invention relates to a process for obtaining sucrose from sugar beets.
The conventional beet sugar manufacturing process involves cleaning the beets, slicing them into cossettes, extracting juice from the cossettes by diffusion, purifying the juice by liming and carbonation, concentrating the juice by multiple effect evaporation, multi-stage boiling of concentrated juice in pans, separation, washing, and drying the sugar.
Juice extraction in the conventional process is done by allowing the sugar to diffuse through the natural cell walls of beets. The cell walls allow sugars and other low molecular weight compounds to pass through but prevent the passage of high molecular weight compounds. This selective diffusion process has two advantages. Retaining the high molecular weight compounds helps produce a high purity juice. It also reduces filtration difficulties that are caused by polysaccharides and proteins that comprise the high molecular weight compounds.
Purification of beet juice in the conventional process is based on lime treatment. Lime serves many purposes in the juice purification process. It neutralizes the acidity of the juice and precipitates calcium salts of several organic and inorganic acids. The precipitate absorbs other impurities. The lime precipitate produces a porous mass, which facilitates subsequent filtration of juice.
The conventional diffusion process for juice extraction from beets has some disadvantages. It has a long retention time, which encourages microbial growth, resulting in sugar loss and formation of undesirable compounds. It is difficult to increase the temperature of sliced cossettes quickly enough to prevent growth of micro-organisms. Typically the pulp remaining after diffusion is pressed and the press juice is introduced back into the diffuser. A significant portion of the high molecular weight compounds retained by the cell walls in the diffusion process is released in pressing to be mixed with the diffusion juice. This partially negates the advantages of the selective diffusion process.
The conventional liming process uses large quantities of lime, amounting to about 2.5% of the total weight of beets processed. Beet sugar plants operate lime kilns and transport limestone over long distances for this purpose. The effluent from the liming-carbonation process, consisting of used lime and separated impurities, is disposed as waste. Production of lime and disposal of liming effluent are costly operations. Disposal of liming effluent is becoming increasingly difficult and expensive in many communities.
Conventional dead-end filtration is incapable of separating sucrose from macromolecular impurities in beet juice. Several methods of using microfiltration and ultrafiltration for purification of juice with reduced lime use have been reported, but these methods generally involve inserting microfiltration or ultrafiltration membranes into the conventional beet process at one or more points.
There is a long-standing need for improved processes for obtaining sugar from beets that avoid or at least minimize one or more of the problems existing in the previously used processes.
The present invention relates to a process for producing sugar from beets. A sucrose-containing feed juice that has been obtained from sugar beets is filtered through a first ultrafiltration membrane that has a first molecular weight cutoff. This ultrafiltration step produces a first ultrafiltration permeate and a first ultrafiltration retentate. The first ultrafiltration permeate is filtered through a second ultrafiltration membrane that has a second molecular weight cutoff that is lower than the first molecular weight cutoff. This second ultrafiltration step produces a second ultrafiltration permeate and a second ultrafiltration retentate. The second ultrafiltration permeate is nanofiltered through a nanofiltration membrane, thereby producing a nanofiltration permeate and a nanofiltration retentate. The nanofiltration retentate has a higher concentration of sucrose on a dry solids basis than the feed juice introduced into the first ultrafiltration step, and can be used in evaporation and crystallization operations to produce crystals of white sugar.
In one embodiment of the invention, the sucrose-containing feed juice is manufactured by macerating sugar beets or pieces thereof, thereby producing a macerated material that comprises pulp and liquid, and then separating the liquid in the macerated material from the pulp, for example by one or more of centrifugation, conventional filtration, or screening. In one particular embodiment, the beets are macerated by first passing them through a hammer mill, and optionally they can subsequently be passed through a grinder, whereby the beets are converted into a mixture of pulp and sucrose-containing liquid. Preferably, no more than about 5% by weight of the sucrose present in the beets remains in the pulp after the liquid is separated therefrom, more preferably no more than about 3%.
After separation of the fibrous pulp from the liquid, and before the first ultrafiltration, the process can optionally include an additional step or steps to remove residual beet fibers and silt from the separated liquid (juice). This can be done by screening and/or filtration. Preferably the screening or filtration removes at least 90% by weight of all fibers and silt having a largest dimension of about 150 xcexcm or greater, more preferably at least 90% by weight of all fibers and silt having a largest dimension of about 50 xcexcm or greater.
It is also possible to introduce air into the feed juice prior to the first ultrafiltration, in order to oxidize color-forming materials. This oxidation, while increasing the color of the juice, causes the color-forming materials to polymerise, which facilitates their removal in the subsequent ultrafiltration. (When this patent refers to polymerisation of color-forming materials, this is intended to include physical agglomeration as well as chemical polymerisation.) Another option is to introduce hydrogen peroxide, ozone, or both, into the feed juice prior to the first ultrafiltration. These materials also facilitate oxidation.
It is preferred to adjust the pH of the feed juice to about 6-8, for example by the addition of a base, prior to ultrafiltration. This can help minimize formation of invert.
The first ultrafiltration membrane preferably has a molecular weight cutoff of at least about 2,000 daltons and a pore size no greater than about 0.1 microns. More preferably, it has a molecular weight cutoff of about 4,000-200,000 daltons. The first ultrafiltration permeate preferably has a color of about 3,000-10,000 icu. (All color values given herein are determined on an ICUMSA scale.)
The process of the present invention can be operated at a number of different process conditions. As representative examples of such conditions, the feed juice can be at a temperature of about 140-200xc2x0 F. during the first ultrafiltration, more preferably about 160-185 xc2x0 F.
The second ultrafiltration membrane preferably has a molecular weight cutoff of about 500-5,000 daltons, more preferably about 1,000-4,000 daltons. In one particular embodiment of the process, the second ultrafiltration is performed in two stages, the first stage using an ultrafiltration membrane having a molecular weight cutoff of about 3,500-4,000 daltons, and the second stage using an ultrafiltration membrane having a molecular weight cutoff of less than about 3,500 daltons. The second ultrafiltration permeate preferably has a color no greater than about 4,000 icu, more preferably no greater than about 2,500 icu.
In order to minimize loss of sucrose in the retentate from the first and second ultrafiltration steps, it is preferable to include diafiltration steps in the process. xe2x80x9cDiafiltrationxe2x80x9d is used herein to mean ultrafiltration that employs added water in the feed to help flush sucrose through the membrane.
In one such embodiment of the process, the first ultrafiltration retentate is diafiltered through at least a first diafiltration/ultrafiltration membrane. This produces a first diafiltration permeate and a first diafiltration retentate. The first diafiltration permeate is then combined with the first ultrafiltration permeate and filtered through the second ultrafiltration membrane.
Similarly, the retentate from the second ultrafiltration can be diafiltered through at least a second diafiltration/ultrafiltration membrane. This second diafiltration step produces a second diafiltration permeate and a second diafiltration retentate. The second diafiltration permeate is then combined with the second ultrafiltration permeate and subsequently filtered through the nanofiltration membrane.
The retentates from the first and second ultrafiltrations (or diafiltrations) and the nanofiltration permeate can be combined to produce molasses. This combined stream may need to be concentrated by evaporation of water.
In addition to purification of the juice by nanofiltration, it is possible to include in the process ion exchange and/or electrodialysis purification steps. These three purification methods can be used in any sequence. In one particularly preferred embodiment of the process, the nanofiltration retentate is purified by electrodialysis, thereby producing a electrodialyzed juice and an electrodialysis residue, and then the electrodialyzed juice is purified by ion exchange, thereby producing a purified juice. Preferably, no lime and no carbon dioxide are contacted with any of the permeates.
The nanofiltration removes ash (including mono- and divalent cations), invert, organic acids, nitrogenous material and other low molecular weight organic or charged compounds. The nanofiltration and the optional electrodialysis and/or ion exchange preferably remove at least about 65% by weight of the Ca, Mg, K, Na and their associated inorganic and organic anions that are present in the second ultrafiltration permeate. The ion exchange replaces remaining divalent cations such as calcium and magnesium with monovalent cations such as potassium and sodium. Preferably, the nanofiltration retentate has a lower concentration of divalent cations on a dry solids basis than the second ultrafiltration permeate.
The nanofiltration permeate will contain a large percentage of the impurities that were present in the feed juice. For example, in many instances, the nanofiltration permeate will comprise at least about 30% by weight on a dry solids basis of the ash, at least about 30% of the invert, and at least about 25% of the betaine present in the feed juice.
The purified juice (i.e., after nanofiltration and any electrodialysis and/or ion exchange), preferably has an ash concentration of no greater than about 2.5% by weight on a dry solids basis, more preferably no greater than about 2%, most preferably no greater than about 1.0%.
After the membrane filtration steps (and any electrodialysis and/or ion exchange), water can be evaporated from the purified juice to produce a concentrated syrup (e.g., 75% dry solids). White sugar can then be crystallized from the concentrated syrup. Because of the high degree of removal of impurities, the present invention can achieve two crystallizations of white sugar from the concentrated syrup, as opposed to one in typical prior art beet processes.
A mother liquor will remain after one or more crystallizations of white sugar from the concentrated syrup. This mother liquor can be recycled to one of the ultrafiltrations. Optionally, this recycle stream can be further purified to reduce its raffinose content.
The process can optionally include sulfitation of one or more process streams. In particular, at least one aqueous stream selected from the group consisting of the feed juice, the first ultrafiltration permeate, the second ultrafiltration permeate, the nanofiltration retentate, and the evaporator feed can be contacted with an agent selected from the group consisting of sulfur dioxide, sulfite salts, bisulfite salts, metabisulfite salts, dithionites, and mixtures thereof, in an amount sufficient to provide an equivalent concentration of sulfur dioxide in the stream of at least about 100 ppm.
One particularly preferred embodiment of the invention is a process for producing sugar from beets that comprises the steps of:
(a) macerating sugar beets or pieces thereof, thereby forming pulp that comprises sucrose-containing aqueous liquid;
(b) separating the sucrose-containing liquid from the pulp;
(c) filtering the sucrose-containing liquid through a first ultrafiltration membrane that has a molecular weight cutoff of about 4,000-200,000 daltons, thereby producing a first ultrafiltration permeate that has a color no greater than about 10,000 icu and a first ultrafiltration retentate;
(d) filtering the first ultrafiltration permeate through a second ultrafiltration membrane that has a molecular weight cutoff of about 2,000-4,000 daltons, thereby producing a second ultrafiltration permeate that has a color no greater than about 4,000 icu and a second ultrafiltration retentate;
(e) filtering the second ultrafiltration permeate through a nanofiltration membrane; thereby producing a nanofiltration permeate and a nanofiltration retentate, wherein the nanofiltration retentate has a higher concentration of sucrose on a dry solids basis than the sucrose-containing liquid in step (b);
(f) purifying the nanofiltration rententate by at least one method selected from the group consisting of ion exchange and electrodialysis, thereby producing an evaporator feed;
(g) evaporating water from the evaporator feed to produce a concentrated syrup; and
(h) crystallizing white sugar from the concentrated syrup.
Optionally, this embodiment of the process can further comprise the steps of:
(i) crystallising a mother liquor from the first crystallisation to produce white sugar;
(j) treating the mother liquor from the second crystallisation by chromatographic separation or by an enzyme to remove raffinose; and
(k) recycling the treated mother liquor back to the nanofiltration feed or the evaporator feed.
Another aspect of the present invention is a process for purifying a sucrose-containing juice obtained from sugar beets. This process comprises the steps of: (a) introducing sufficient air into the juice to cause polymerisation of color bodies; and (b) removing at least some of the color bodies from the juice by membrane filtration through at least one ultrafiltration membrane or nanofiltration membrane.
The various aspects of the present invention have a number of advantages over prior art beet processes. For example, the process of the present invention eliminates the need for a lime kiln, lime quarries and all associated equipment, processes, products, by-products and waste products. Also, the present invention results in a drastic reduction of waste products that cause environmental pollution. The conventional process produces a filter cake that comprises products of the liming process and impurities removed from the juice. This cake is disposed into ponds or landfills. The proposed process completely eliminates the need for disposal of such materials. The present invention allows elimination of the carbonation process, which is a major source of atmospheric pollution in beet sugar plants.
The present invention provides a cost-effective way of reducing the ash content of the beet juice or syrup, preferably to about 2% or less (on a dry solids basis), more preferably to about 1.5% or less, most preferably to about 1% or less. This reduction in ash content is important because it allows a second strike of sucrose crystals from the syrup. In prior art beet processes, ash contents in the range of 3.5% made it practically impossible to have more than one strike of sucrose crystals.
In addition, the present invention can eliminate the need for desugarization of molasses streams. The efficient membrane filtration steps prevent excessive amounts of sucrose from entering the molasses streams in the first place.
Further, the present invention provides an economical and reliable method for removing color-causing materials from beet juice. It also can reduce the formation of undesirable crystalline forms due to the presence of excessive amounts of raffinose.